The present invention relates generally to the measurement of mass flow rate. More particularly the invention provides an inferential fuel mass flow rate measurement and control system in which the density of the fuel is precisely measured by a unique fluidic liquid density sensing device. The fluidic device has a very high degree of accuracy which is unaffected by variation in the temperature, pressure or viscosity of the fuel.
As the fuel economy and performance requirements of modern aircraft jet propulsion engines continue to increase, the need to precisely monitor and control the fuel delivery mass flow rates to their gas generators and augmentors has become far more critical than in the past. Conventionally, two approaches have been taken to the mass flow rate measurement of jet engine fuel delivery. Neither has proven wholly satisfactory.
The first approach has been to "inferentially" measure the fuel's mass flow rate by separately measuring its volumetric flow rate and its density, and then combining (by multiplication) the two measured parameters to provide an indication of mass flow rate. Using this indirect approach, volumetric flow can be measured with acceptable accuracy by any of several conventional flowmeters, including turbine, differential pressure, positive displacement, ultrasonic or fluidic devices. However, conventional methods of measuring the fuel's density in the inferential system inpart heretofore unavoidable inaccuracies to the system.
As an example, perhaps the simplest conventional method of measuring liquid density comprises measuring the temperature of the liquid and then obtaining its density via the temperature-density relationship for the particular liquid. While this method is suitable for a single component liquid, it has proven unsatisfactorily inaccurate in the case of multi-component engine fuel due to unavoidable batch variations in such fuel. Additionally, many advanced propulsion engines are required to be operable on a variety of fuel types--an operating characteristic far beyond the capabilities of conventional temperature-density sensing apparatus.
More complex methods of liquid density determination include electrically driven vibrating beam densitometers and devices which measure the dielectric constant of a liquid, the dielectric constant being related to the liquid's density. Inherent in both of these devices, however, are rather poor accuracy and slow frequency response.
The second conventional approach to determining the mass flow rate of a liquid is to measure it directly by utilizing a "true" mass flowmeter. The most widely used device of this type, the angular momentum mass flowmeter, uses a motor-driven or fuel-driven impeller which imparts angular momentum to the fuel at a rate of proportional to the fuel's mass flow rate. The fuel is then flowed through an identically configured turbine which is rotationally restrained by a spring. By electronically measuring the angular deflection of the turbine the mass flow rate of the fuel is directly obtained.
Such conventional true mass flowmeters afford the advantage of being substantially unaffected by variations in the fuel's physical characteristics such as temperature, pressure and viscosity. Thus, a single such mass flowmeter has the potential for accurately measuring the mass flow rate of a variety of engine fuel mixtures. However, the requisite degree of measurement accuracy needed for high technology turbine engines may be achieved only by very precisely manufacturing both the impeller and turbine elements--a very costly process. Additionally, this type of flowmeter has demonstrated the requisite accuracy only within the "cruise" range of the typical turbine propulsion engine. Outside of this range the flowmeter's accuracy falls off markedly.
It can be seen from the foregoing that a need exists for a fuel mass flow rate measuring and control system which eliminates or minimizes above-mentioned and other problems associated with conventional apparatus. Accordingly, it is an object of the present invention to provide such a system.